In a magnetically shielding structure constructed by a superconductor, Meissner effect is utilized for the magnetic shield. That is, a material having Meissner effect is, for example, formed into a cylinder shape to form a shielding body and it is cooled below the critical temperature Tc for the transition to a superconductive state for making the shielding body a diamagnetic thereby a magnetic flux is forced out to the exterior of the shielding body, and the internal space of the shielding body is magnetically shielded.
On the other hand, in the case of a shielding structure with a high permeability material being commonly used without utilizing a superconductor, where a shielding body is formed into a cylinder with the high permeability material, for example, and if the shielding body is held in a magnetic field, magnetic induction is generated in the wall of the shielding body and the magnetic field is short-circuited along the shielding body, thereby the internal bore space of the cylinder is magnetically shielded.
In such a magnetically shielding structure utilizing a superconductor, although the magnetically shielding capability of a cylindrical shielding body, for example, is high enough for a magnetic field parallel to the center axis of the cylinder (longitudinal magnetic field), yet the magnetically shielding capability for a magnetic field perpendicular to the center axis (lateral magnetic field) is not enough. Therefore, there is a problem that the length of the cylinder has to be long in comparison with the inner diameter of the cylinder.
On the other hand, in the case of a cylindrical shielding body formed with a high permeability material, for example, the shielding capability for longitudinal magnetic field is not enough in comparison with that for lateral magnetic field, and therefore, to obtain high shielding capability, a plurality of cylinder walls have to be laminated and a structure in which the inner layer is shorter than the outer layer has to be adopted. In the result, even in the case of a high permeability material being used, there has been a problem that the length of a cylinder at outermost layer becomes long, and when a usable space is to be larger the length has to be longer. Further, since the shielding effect of a high permeability material for an inside feeble magnetic field depends upon the initial specific permeability thereof, the shielding capability is not so enough, and for improving shielding capability the shielding body has to be made to have a multilayer structure, which causes a problem that the shielding body becomes expensive.